A lookup service contains an indication of where network services are located within a distributed system comprised of multiple machines, such as computers and related peripheral devices, connected in a network (for example, a local area network, wide area network, or the Internet). A "network service" refers to a resource, data, or functionality that is accessible on the network. Typically, for each service, the lookup service contains an address used by a client (e.g., a program) to access the service (e.g., a printer).
Conventional lookup services are static: whenever updates to the lookup service are needed to either add a new service or delete an existing service, the lookup service is taken offline, rendering the lookup service inaccessible, and then, the lookup service is manually updated by the system administrator. During the time when the lookup service is offline, clients in the distributed system are unable to access the lookup service and any of its network services.
Another limitation of conventional lookup services is that, when updated, clients are not made aware of the updates to the lookup service until they explicitly perform a refresh operation, which downloads the latest service information to the clients. Before such a refresh, however, if a client requests a service that is no longer available, an error occurs which may cause the client to hang. Also, before a refresh, the client is not aware of any new services that have been recently added to the lookup service. It is therefore desirable to improve lookup services for distributed systems.